The present invention concerns a diffuser for a single-impeller centrifugal pump, particularly suited to be advantageously applied on plastic self-priming swimming pool pumps or whirlpool pumps and to self-priming ejector pumps, usually known as "jet" pumps.
It is a known fact that a self-priming pump is a pump for liquids which, when started, can suck by itself the air which is present in the suction piping, after the pump casing alone has filled up with liquid. A pump is primed when the waterways of the pump are filled with the liquid to be pumped.
It is also a known fact that in the single-impeller centrifugal self-priming pumps there is a diffuser, i.e. a duct having a cross-section which increases in the direction of the flow, which has the task of transforming as much as possible the kinetic energy of the liquid streaming out of the impeller into pressure energy.
The single-impeller centrifugal pumps of the above-mentioned types include a bladed diffuser having a round shape and radial outlets, which is arranged around the periphery of the impeller in the pump casing such a diffuser is positioned within an annular chamber closed around the periphery and axially open for the "jet" pumps, or within a chamber open both radially and axially in the swimming pool pumps. The known type of diffusers having blades with the mentioned radial outlets present, however, some inconveniences.
A first inconvenience arises from the fact that the pumped liquid flows out of the diffuser with a tangential or a helicoidal direction and, during the priming phase, it drags along the air contained in the pump casing. The centrifugal effect separates the air from the liquid. The air has a tendency to collect around the axis of rotation in the pressure chamber or to flow back radially through the diffuser to the hub of the impeller and it arrives with difficulty at the delivery connection. For this reason the pumps built according to the known techniques require extensive longitudinal baffles or other devices, in order to slow down the rotation in the pressure chamber or they require manually operated adjustment devices such as throttle valves or methods of topping up with water through the delivery pipe, in order to obtain the priming with the maximum suction lifts.
Another inconvenience consists in the fact that the presence of the radial diffuser causes an increase in the radial overall dimensions of the pump, since the blades are usually, although not always compulsorily, overlapped between the inlet and the outlet, in order to obtain a diffuser duct. This requires around the diffuser a relatively high circular rim, so that the outer diameter of the pump equals the diameter of the impeller plus the height of the diffuser, plus the height of a possible annular free chamber, plus the thickness of the pump casing.
In the swimming pool pumps or whirlpool pumps with a traditional radial diffuser, the height of the axis of the pump casing exceeds the height of the motor axis. Because of the weight differences of the plastic materials of the pump and the metal materials of the motor, the center of gravity of the unit is under the motor, in spite of the larger volume of the pump casing. In order to hold the unit in a horizontal position, these pumps require an added support consisting of one or more parts fastened with screws which, when applied under the motor, must exceed the axis height of the pump or, when applied under the pump extends down to the center of gravity under the motor. This results in further inconveniences relating to the stability and the structural sturdiness, while entailing some complications for the manufacture and the assembly, and because of the increase in weight and costs.
Moreover, in the mentioned swimming pool pumps or whirlpool pumps, the impeller and the diffuser must be placed within the pressure chamber of the pump casing and this entails one more inconvenience, i.e. it decreases the useful volume for the amount of water with which the pump casing can be filled up at the start for the self-priming process.
Some "jet" pumps are known and they were disclosed by the patents DE/3718273, U.S. Pat. Nos. 2,941,474 and 2,934,021, wherein, in the attempt of solving the problem of the rotation and of the vorticity of the liquid and the air at the outlet of the diffuser, a separation wall or an added suitable component, such as a separating device or a perforated screen, is arranged behind the diffuser, in order to transversally divide the pressure chamber into a chamber with high vorticity and high rotational speed and a chamber with less turbulence and a decreased rotational speed.
Moreover, both "jet" pumps and plastic swimming pool pumps or whirlpool pumps are known, wherein the rotation of the liquid and the air within the pump casing is slowed down by means of extensive longitudinal baffles. These devices present, however, the inconvenience that they do not eliminate the origin of the vorticity and of the rotation of the liquid and the air, but they only intercept them in order to decrease their intensity with negative effects on the efficiency and on the noise level. The obstacles used to slow down the rotation present yet the further inconvenience that they also increase the friction on the water in the pump casing, so that the temperature of the water reserve contained in the pump casing increases in a shorter period of time.
This limits the self-priming capacity and reduces the period of time during which the pump can function without being damaged, if the water reserve is not renewed because of accidental causes, as when the pump keeps on running with a closed gate valve or with the suction pipe not immersed in the water because of the lowered water level of the pool.
Another known "jet" pump is the one disclosed in the patent application EP0361328 by the same inventor which presents a diffuser of the axial type arranged in front of the impeller on its suction side, with some return channels converging to a central exit, without any rotation of water and air within the pressure chamber. Although this axial diffuser eliminates the inconveniences of the radial diffuser in the above-described applications and practically fulfils the same purposes of the present invention, it presents the inconvenience for the manufacturer that it requires two pieces (added between the pump casing and the casing cover) with a complicated blades system. It also requires the addition of a metal wear ring, in order to protect against abrasive substances the clearance of the non-contact seal ring on the impeller, when the impeller and/or the diffuser are made of plastic material. Said axial diffuser also presents the inconvenience that, if it is made only with the single central exit, it leaves in its interior and on the impeller side an air pocket when it is filled up for the first time and a water pocket when it is drained.